MPC5601PEF0MLH6R datasheet(57/95 Pages) FREESCALE

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MPC5602P Microcontroller Data Sheet, Rev. 4.1

Freescale Semiconductor

Figure 13. ADC characteristics and error definitions

3.14.1

Input impedance and ADC accuracy

To preserve the accuracy of the A/D converter, it is necessary that analog input pins have low AC impedance. Placing a capacitor

with good high-frequency characteristics at the input pin of the device can be effective: the capacitor should be as large as

possible, ideally infinite. This capacitor contributes to attenuating the noise present on the input pin; it sources charge during

the sampling phase, when the analog signal source is a high-impedance source.

A real filter can typically be obtained by using a series resistance with a capacitor on the input pin (simple RC filter). The RC

filtering may be limited according to the source impedance value of the transducer or circuit supplying the analog signal to be

measured. The filter at the input pins must be designed taking into account the dynamic characteristics of the input signal

(bandwidth) and the equivalent input impedance of the ADC itself.

In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: CS being

substantially a switched capacitance, with a frequency equal to the ADC conversion rate, it can be seen as a resistive path to

(2)

(1)

(3)

(4)

(5)

Offset Error (EO)

Gain Error (EG)

1 LSB (ideal)

1023

1022

1021

1020

1019

1018

1017 1018 1019 1020 1021 1022 1023

(1) Example of an actual transfer curve

(2) The ideal transfer curve

(3) Differential non-linearity error (DNL)

(4) Integral non-linearity error (INL)

(5) Center of a step of the actual transfer curve

1 LSB ideal = VDD_ADC / 1024

Vin(A) (LSBideal)

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